The present invention relates to a coding apparatus for a moving object image signal and, more particularly, to a coding apparatus for a moving object image signal wherein high coding efficiency and high image quality can be provided for an uncovered background appearing from behind a moving object on a screen.
A conventional motion compensated interframe coding scheme has been used to compress a television signal or the like with high efficiency and to transmit the compressed television signal. According to this scheme, as described in U.S. Pat. No. 4,307,420 (Dec. 22, 1981) issued to Ninomiya et al, a bandwidth can be effectively compressed by utilizing interframe correlation of a moving object image signal.
Another conventional scheme for further effectively compressing a bandwidth is a combination of predictive coding using interframe correlation and coding using intraframe correlation (to be referred to as a hybrid coding scheme hereinafter), as described in IEEE Transactions on Communications, Vol. COM-29, No. 12, December 1981, pp. 1799-1808. In general, the screen includes a still image area such as a background, a moving image area of a moving object, and an uncovered background area appearing from behind the moving object. In the uncovered background area, interframe correlation cannot be established. Thus, the conventional interframe predictive coding and the hybrid coding scheme cannot provide satisfactory coding efficiency.
In a conventional coding scheme using interframe correlation, an arrangement shown in FIG. 1 is employed to reduce noise. Referring to FIG. 1, a nonlinear circuit 1 having nonlinear characteristics is used. In this circuit, if an interframe image signal difference is small, the gain is not larger than 1 (e.g., 1/4to 1/2). However, if the interframe difference is large, the gain becomes 1. In the succeeding two frames, between which an object is moving, a background portion behind the moving object in the first frame appears in the second frame. In the conventional noise reduction circuit in FIG. 1, even a small interframe difference is detected as noise, and the gain of the nonlinear circuit is set to be less than 1. A signal distortion appears on the screen, i.e., providing a tail following the moving object, thus resulting in a poor image.